1. Field of the Invention
This invention relates generally to patterned perpendicular magnetic recording media, such as disks for use in magnetic recording hard disk drives, and more particularly to patterned disks with uniform data islands having improved magnetic recording properties.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In conventional continuous magnetic recording media, the magnetic recording layer is a continuous layer over the entire surface of the disk. In patterned media, also called bit-patterned media (BPM), the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks. While BPM disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer, will likely be the choice for BPM because of the increased data density potential of perpendicular media. To produce the magnetic isolation of the patterned data islands, the magnetic moment of the spaces between the islands is destroyed or substantially reduced to render these spaces essentially nonmagnetic. Alternatively, the media may be fabricated so that there is no magnetic material in the spaces between the islands.
Nanoimprint lithography (NIL) has been proposed to form the desired pattern of islands on BPM disks. NIL is based on deforming an imprint resist layer by a master template or mold having the desired nano-scale pattern. The master template is made by a high-resolution lithography tool, such as an electron-beam tool. The substrate to be patterned may be a disk blank with the magnetic recording layer, and any required underlayers, formed on it as continuous layers. Then the substrate is spin-coated with the imprint resist, such as a thermoplastic polymer, like poly-methylmethacrylate (PMMA). The polymer is then heated above its glass transition temperature. At that temperature, the thermoplastic resist becomes viscous and the nano-scale pattern is reproduced on the imprint resist by imprinting from the template at a relatively high pressure. Once the polymer is cooled, the template is removed from the imprint resist leaving an inverse nano-scale pattern of recesses and spaces on the imprint resist. As an alternative to thermal curing of a thermoplastic polymer, a polymer curable by ultraviolet (UV) light, such as MonoMat available from Molecular Imprints, Inc., can be used as the imprint resist. The patterned imprint resist layer is then used as an etch mask to form the desired pattern of islands in the underlying magnetic recording layer.
It is important in BPM that there be uniformity in size and magnetic properties among the data islands. The switching field distribution (SFD), i.e., the island-to-island variation of the coercive field, needs to be narrow enough to insure exact addressability of individual islands without overwriting adjacent islands. Ideally the SFD width would be zero, meaning that all the bits would switch at the same write field strength. Variations in the size, shape and spacing of the islands, as well variations in the composition and crystallographic orientation of the magnetic material, increase the SFD.
It is also important to improve the writability of the individual data islands. It may desirable for the recording layer in the islands to be formed of at least two ferromagnetically exchange-coupled magnetic layers having different anisotropies to provide a graded anisotropy across the thickness of the recording layer. This type of graded-anisotropy recording layer compensates for the variation in write field across the thickness of the recording and non-uniformities in the write field gradient.
The uniformity in size and magnetic properties among the islands and the writability of the individual islands can be adversely affected by the etching process. Thus it is also important to be able to protect the individual magnetic layers in the islands during the etching process.
What is needed is a patterned perpendicular magnetic recording with uniform data islands having improved writability.